1. Field of the Invention
The present invention relates to a fluorescent lamp with a resonant type circuit driven by a half-bridge circuit and, more particularly, to a circuit for detecting near or below resonance operation of such a lamp.
2. Description of the Related Art
FIG. 1 shows a typical lamp resonant output stage being driven by a half-bridge circuit including MOSFETS M1 and M2 providing current I.sub.L1 through inductor L1 to the lamp connected in parallel with capacitor C1. A sense resistor R.sub.CS is connected between the source of lower MOSFET M2 and ground. As shown in the timing diagram of FIG. 2, under normal operating conditions, the phase of the inductor current I.sub.L1 with respect to the half-bridge voltage V.sub.S lies somewhere between 0 and -90 degrees. Should the phase approach 0 degrees, however, the frequency is approaching resonance. At or near resonance, non-zero voltage switching can occur at the half-bridge, resulting in a large current spike at turn-on in either of the two half-bridge switches, as shown in the timing diagram of FIG. 3.
Referring to the transfer function for running and before ignition conditions depicted in FIG. 4, it is also possible for the resonant lamp output stage to be operating above the resonance frequency of the low-Q circuit (during running), but below the resonance frequency of the high-Q (before ignition) circuit. If the lamp is then removed, the transfer function jumps from the low-Q to the high-Q curve while the frequency remains unchanged and below the resonance frequency of the high-Q circuit. This results in almost immediate destruction of the half-bridge.
Another condition which can cause below resonance operation is if the filaments of the lamp are intact, but the gas within the lamp escapes (e.g., the glass cracks). In this condition, the load operating condition would instantaneously change from the damped (above resonance) to the undamped (below resonance).
Accordingly, it would be desirable to be able to detect of operation of the lamp near or below the resonance frequency, and to shutdown operation of the lamp under such conditions to prevent catastrophic failure of the switching devices (usually MOSFETs) of the half-bridge driver circuit.